A spinning pack used to discharge and mold synthetic resin is generally preheated to near operating temperature before being attached to a melt spinning apparatus. In many cases, a preheating furnace is used to heat the spinning pack to a high temperature around 300° C.; however, it takes time to reach a desired temperature.
For this reason, as described in Patent Literature 1, it is well known that an induction coil may be wound around an outer circumferential surface of a spinning pack to raise a temperature of the spinning pack in a short period of time by induction heating.
However, in the case of winding the induction coil around the outer circumferential surface of the spinning pack, there is a problem that the temperature of an outer circumferential part of the spinning pack rapidly rises, and therefore it takes time for a temperature of the inside, in which a flow path through which molten resin passes is formed, to reach a desired temperature.
Also, in many cases, inside of the spinning pack, a filter for removing foreign substance in the molten resin is provided, and in terms of material and structure of the filter, it is desired to set a maximum allowable temperature to approximately 400° C. For this reason, in the case of performing the heating until the inside temperature reaches the desired temperature, there is a problem that the temperature around the inside exceeds the maximum allowable temperature to damage the filter.